This invention relates to producing fine fibers from thermoplastic material, such as but not limited to glass. More specifically this invention involves improvement in the production of fine fibers by modifying conventional flame blown and/or rotary processes and/or apparatus for producing fine fibers.
The conventional "flame blown" process comprises drawing molten glass from a furnace and continuously cooling said molten glass to form one or more rods or "primary fibers" generally having diameters in the range on the order of 0.010 to 0.035 inch. These primary fibers are continuously fed side by side into a high temperature high velocity blast of gas that extends sidewise with limited height to form a wide blast that intercepts the primary fibers. The high temperature high velocity blast softens the glass rods and the force of the blast attenuates or elongates the softened material into fibers.
The production efficiency of this fiberizing process is dependent on how fast the primary blast of hot gas can soften the primary fibers and how long a time the primary fibers can be stretched or attenuated.
Typically, primary fibers are aligned on approximately 0.060 inch centers and fed into the primary gas blasts at approximately 0.40 pounds per hour per primary fiber. These primary fibers can be attenuated to an average fiber diameter of about 0.00015 inch.
With production rates conventional in the prior art and knowing the density of the glass, one can calculate that the attenuating ability of the blast is approximately 5800 feet per second. The attenuation rate, in this example 5800 feet per second, is the heart of the prior art process and governs the production rate (pounds per hour) and the economics of the operation.
In the remainder of this discussion, the rate of attenuation will be called Specific Attenuation Rate (SAR) and is by definition the mathematical length of a round fiber whose volume is equal to the volume per second that is softened from each primary fiber. ##EQU1## where M=pounds/hour/primary fiber fed to the primary blast
D=average diameter of the final fiber (inches) PA1 32.647.times.10.sup.-5 =constant adjusting for units and glass density, etc.
In the "flame blown" process, batch material or marbles are melted in a furnace and directed to a forehearth above the fiber forming equipment. In the floor of the forehearth containing the molten glass are several "bushings" that are positioned in a line. Each "bushing" is a device generally fabricated of a platinum-rhodium alloy with several series of holes or tips through which the molten glass flows. The molten glass flows through these tips at a rate governed by the hole size, length of tip, head of glass over the tip, and viscosity of the glass at the time it is flowing through the tip. This glass then cools by radiation and convection and by its own weight will be drawn into a round rod called a primary fiber. The diameter of these primary fibers is controlled by the glass composition, bushing temperature and a force applied by cot rolls which exert a uniform rate of downward pulling. These primary fibers are for example pulled in a line on approximately 0.060 inch centers and directed into a high velocity high temperature flame. This flame is blasted at nominally but not necessarily right angles to the line of primary fibers.
In order to produce the high velocity flame blast, an unburned combustion mixture is fed along a feeder pipe to a burner. Combustion takes place in a combustion chamber within the burner. The latter has an elongated horizontally extending slot opening. The high temperature products of combustion leave the burner in the form of a blast along a primary axis at a high velocity, in general 400 to 800 feet per second and at a temperature on the order of 3000 degrees Fahrenheit or higher. This high temperature, high velocity blast (typically 1/4 to 3/4 inch vertically by 4 to 16 inches horizontally) is directed along a primary axis that intersects the primary fibers which are being forced in a downward direction into the blast. The primary fibers are heated due to the blast and when the fibers soften, the force of the blast bends the primary fiber and begins to lengthen them to form attenuated fibers. The softened primary fibers continue to soften as they get hotter and the force of the blast tends to pull them away from the point of bending along the primary axis. If the fibers are pulled in a straight line and at the speed of the blast, one would be producing fibers at a Specific Attenuation Rate (SAR) of 400 to 800 feet per second depending on the speed of the blast.
SAR rates of 5800 feet per second are not at all uncommon in actual operation. The reason for the increased attenuation over the blast velocity can be explained by turbulence.
Turbulent motion is characterized by a random motion of particles constituting the fluid stream and in general makes its appearance in fluids when they flow past solid surfaces or when neighboring streams of the same fluid flow past one another. Irregularity or randomness of the fluctuations is essential for turbulent motion. A fluid in turbulent motion may be thought to possess an average velocity in the (x) direction along a primary axis with a superimposed random motion resulting in instantaneous deviations from the average velocity at any point. Thus, a fine dust particle in the stream would be seen to describe an irregular zigzag motion in all directions, while being carried downstream along the direction of the primary blast at an average velocity. The instantaneous fluctuating velocity at any point has components in x, y, and z directions. The root mean square values of these velocities is referred to as the intensity of turbulence.
There are two other concepts of turbulent motion that should be briefly mentioned in line with the intensity of turbulence. These are defined by scales (1.sub.1) and (1.sub.2). The scale of turbulence (1.sub.1) is analogous to the mean kinetic free path of molecules that is associated with Brownian movement. Grossly simplified, that would be the average path length the zigzagging particle mentioned above would travel before changing direction. Another scale (1.sub.2) is based on the observation of the fluctuating velocity in the (x) direction at two points in the stream separated by a distance (y) in the (y) direction. If the points are very close together the observations will be identical, that is they will show perfect correlation of the flow, and if sufficiently far apart no correlation will be found. By definition the scale (1.sub.2) may be regarded as proportional to the average size of an eddy. There is no theoretical relation known between (1.sub.1) and (1.sub.2).
In the conventional "flame blown" process the primary fibers are first softened by the temperature of the blast and then attenuated by the high velocity of the hot blasts arranged in a horizontal row. The attenuation in each blast is augmented by the turbulence in the blast. As the hot blast entrains surrounding air, the turbulence is increased somewhat due to the mixing, and the temperature is lowered, also due to the mixing with the colder surrounding gases (in general, air). The fiber will be continuously attenuated as long as it remains at a sufficiently high temperature (or low viscosity) so that the turbulent energy of the hot gases can attenuate or stretch each small section of fiber.
With this reasoning, one can see why the SAR can be greater when the end product is desired to be a very fine fiber--say 0.00003 inch diameter versus a 0.00015 inch diameter fiber. Less energy and smaller scales of turbulence are needed to further attenuate finer fibers that would be insufficient to attenuate larger cross-sectional fibers.
In general, attenuation takes place when the ratio of viscosity to surface tension is between 5 and 50. If this ratio is much below 5, the surface tension force is strong enough to form a sphere creating what is referred to as "shot" instead of a fiber. When the ratio exceeds a value in the neighborhood of 50, the viscosity is so great the turbulent blast is unable to stretch or attenuate the fiber additionally.
In the conventional rotary process where molten glass is fed through a rotating apertured wall by centrifugal force and the resulting fibers are engaged with high speed, high temperature blasts from an encircling ring burner, the SAR values are roughly equal to or lower than what has been obtained with the flame blown process. The major economic advantages of the various rotary processes are two-fold:
1. molten glass is attenuated directly into fibers, eliminating the energy needed to reheat the primary fibers; and,
2. many more streams of molten glass can be assembled into a given space than is possible with the flame blown system which produces primary fibers along one or more rows at a spacing of 0.060 inch.
As mentioned earlier, the flame blown process can only handle one linear row of primary fibers into the blast on approximately 0.060 inch centers (approximately 200 fiberizing sources or primary fibers per linear foot).
In contrast to this, the rotary process can develop primary fibers from 3000 to 50,000 holes in a single spinner of 12 inch diameter. This would be approximately 1000 to 15,000 fiberizing sources per linear foot. However, the SAR is in general considerably less than in the flame blown process. At best it is equal to the flame blown process in SAR.
Currently the state of the art limits production of fine fibers to a maximum SAR of about 30,000 feet/second.
The following table puts this in focus and shows what could be done to increase the rate of fiber production with the present invention if the present invention could develop higher SAR values without introducing problems that existed in the prior art.
TABLE I ______________________________________ Fiber Diameter Fiber Diameter Pounds Per Hour Inches Microns SAR Per Primary Fiber ______________________________________ 0.00015 3.81 5,800 .40 0.00003 .762 15,000 .041 0.00005 1.27 30,000 .23 .00004 1.00 30,000 .145 0.00003 .762 30,000 .083 .00002 .508 30,000 .037 .00001 .254 30,000 .009 .00001 .254 40,000 .012 .00001 .254 100,000 .030 .00001 .254 200,000 .061 .00001 .254 300,000 .092 ______________________________________
One can see that to make a given diameter fiber the production rate is directly proportional to the SAR. There is a market for fibers of 0.25 microns if they can be produced at a lower cost. Currently, 1.27 micron fibers sell for about $3.00 per pound at production rates of approximately 0.23 pounds per hour per primary fiber and 0.25 micron fibers can be produced at 0.009 pounds per hour or approximately 4% the production rate of the 1.27 micron fibers. Such slow production makes the selling price of the finer fiber prohibitive at 30,000 SAR, the limit of current technology.
Prior to this invention, the maximum SAR that could be developed with the "flame blown" or the rotary process was limited to approximately 30,000 feet per second. The preceding table shows that the production rate to produce fibers having a diameter of 0.25 microns was limited to 0.009 pounds per hour per primary fiber. With modifications to the flame blown apparatus, SARs have been developed in the range of 100,000 to 300,000 feet per second. This means that it is possible to produce attenuated glass fibers of 0.25 microns diameter at a rate of feed of 0.092 pounds per hour per primary fiber using prior art flame blown apparatus incorporating an enclosed duct having a suddenly enlarged downstream portion and walls heated to a temperature within the glass attenuation range. However, attenuated glass fibers stick to hot duct walls and production has to stop to clean the ducts too frequently to develop an economical operation because the time needed to clean the ducts is nonproductive. Also, the ratio of length to diameter of the thinner attenuated fibers, particularly those of submicron diameter developed at higher SAR values, is too small to fabricate fiber glass insulation pads of acceptable flexibility and tensile strength. The ratio of length to diameter of submicron fibers should preferably exceed about 200 for ultrafiltration uses. This ratio should preferably exceed about 2,000 for thermal insulation purposes where fibers coarser than one micron diameter are involved.
The prior art recognized the limitations of a fiber forming system that included a single attenuation means and developed systems with auxiliary glass attenuation means. Despite many attempts to be described in connection with a discussion of patents of interest that follows, prior art patented systems provided with auxiliary attenuation means failed to solve the aforesaid problems.